As is known, Winchester-type fixed, rigid disk drives have recirculating air flow systems and have relatively little tolerance to the presence within such air flow of particulate material as small as a fraction of a micron because such particles may cause "head crashes" or otherwise damage the heads and/or disks. Indeed, the current industrial standard specification demands that the recirculating air contain no more than about 200 particles having a diameter of 0.3 microns or larger per cubic foot of air under steady state conditions. As will be appreciated, that is equivalent to a "Class 100" clean room environment.
Winchester disk drives conventionally are assembled and sealed in clean rooms, but an unacceptably high level of particulate contaminents is almost invariably introduced into the sealed enclosure or "bubble" of such a disk drive during the assembly process. Thus, the accepted practice is to include an internal air filtration system within such a disk drive for bringing the level of particulates down to an acceptably low level during an initial "clean-up" operation of the disk drive. The drive is at risk of a potentially catostrophic failure on account of an excessively high contaminent level while the clean-up operation is being performed. Thus, it is noteworthy that prior filtration systems typically have required as long as ten minutes or so to complete the clean-up process.
There are at least three distinct varieties of internal, pass-through air filtration systems for Winchester-type disk drives, each with its own advantages and disadvantages. Some drives, such as the Shugart 1000, have a filter mounted within a sealed plenum through which the recirculating air is forced to pass. This so-called "forced loop" approach is attractive because the air does not easily bypass the filter, but it has the disadvantages of requiring extra space, being relatively costly to implement, increasing the power consumption of the drive, and requiring careful sealing of the plenum. Other drives, such as the Shugart 1100, develop relatively high and relatively low static pressures at the inlet and outlet, respectively of the filter, so that the recirculating air is urged through the filter without the benefit of a sealed plenum. This "push-pull" approach is less efficient than a forced loop air filtration system because air can easily bypass or "blow-by" the filter, but it requires less space, is less costly to implement, reduces the power consumption and avoids the plenum sealing problems. Still other drives, such as the Seagate 412 and 506, simply develop a relatively high static pressure at the inlet of the filter for pushing the recirculating air through the filter. This "push-only" approach is the least efficient air handling technique, but it is a workable solution for drives involving certain spatial constraints.
A common characteristic of the existing pass-through air filtration systems for Winchester disk drives is that they employ a highly efficient mechanical filtering medium, even though such a filter inherently has substantial air flow resistance. For example, in view of the current state of the art, a conventional specification for such a filtration system would typically require a fiberglass filtering medium having a 0.3 micron mechanical filtering efficiency of approximately 99.98%. Lower resistance, adhesion-type air filtration systems have been proposed for Winchester disk drives, such as in U.S. Pat. No. 4,001,892, which issued Jan. 4, 1977 on "Contaminant Trapping in Rotating Disk Devices," but nothing has been uncovered to suggest that disk drive designers have previously recognized that a relatively low efficiency filtering media can be used in pass-through air filtration systems.